Recovery of precious metals from photographic baths



Oct. 28, 1952 H. o. WILLIER 2,615,839

RECOVERY OF PRECIOUS METALS FROM PHOTOGRAPHIC BATHS Filed April 12, 1948 m 9 WW HQFVz'ZZz'er 'ing of the bath.

the bath-or solution.

'In the drawings, Figure 1 depicts a vertical; sectional view wherein'is shown a tank 8;; for

Patented Oct. 28, 1952 STATES RECOVERY OF PRECIOUSLME Ans FROM-.1 1

PHOTOGRAPHIC'BATHjS" Henry).WillieyBethesda pplication April 12, meta-s r al excess-7 .3 Claims, (o1. '204 Fixing photographic baths or solutions for photographic films, plates and the like are usually made of a Solution of sodium thiosulfate, or ammonium thiosulphate to which certain other chemicals are added. It isdesirable to remove the silver which accumulates therein after a period of time of use of the bath, this for the value of-th'e metal itself and to prolong the useful' life of the bath.

My method of removing the silver is to-subject the bath to an electrolysis treatment in a manner v as hereinafter described and with certain specific highly advantageous means. 7 h Th's invention concerns itself therefore with a novel method and apparatus for recovery of preciousmetals, more particularly silver ffrom photographic solutions or baths. {In prior methods of electrical silver recovery very special and onerous precautions are usually observed as to voltages; 'or rapid agitation of the bath,- or additions of special chemical-activators are resorted to. The silver' can be recovered on a'solid cathode plate-but it" forms a spongy 'po'rous mass which-is so weakly adherent tothe plate that the slightest jar which usually occurs in working with the solutioneaus'es the silver to fall off to the'bottom of the tank and there is strong tendency to current density foulj I'Io keep the adherent silver coating onv the cathode smooth and not spongy prevents entrapment of pockets of the solution in the silver and prevents consequent fouling of the solution;

Ihave discovered that by'suspending at one side ofga fixing bath tank an anode plate, made diffi culties heretofore described-besides getting out more of the silver and prolongingthe life'of containingua photographic fixing bath or soluthe level of solution being indicated by the broken li-nes,alt-houghfit will be understood this level is variable according to ordinary practice. 'Suspended'at one side of the tank 8, as by a hanger II, which may be of stainless steel or any strong materialynot; eife'cted by the hypo solution, is

an anodeplate 12', the hanger ll passing throughasmaueepening? magma. Arleadwire tola "tion and having-side walls a, 'anda bottom 10,

sourceof'current supply is indicatedat M; The

anode platelex-tends almost to,- the'bottom; ofthe ,tank and' is slightly-spaced from the; adj acent'side wall:9;r At the other side of the, tank is acathode I 5, having a suitable leadi'wire; 15a, which in this case issupported by. bending aiportion= l6 thereof over the .top edge of the tankwall 9.. 'I'hiscathode'isiformed of a metalysuch as stainlessnsteel or Monel metal or any electrically conducting material whichis resistant to attack by the bath or solution.- i It is mostrconveniently made of wire mesh similar to screen .wire, an enlarged detail being. shown in Figure 4. The cathode I5: is of several layers of themesh' or screen, bent-or reverted worsen :the other as seen; in Figure, 1 to there'bmlpresenta. large working surface area an elevational view of the anode [2 which may be of graphite or any :other suitable material for conducting an electric current. .In Figure .3

is'showna fragmentof the screen wire or mesh cathode I5. Figure 5 is a-modified form of cathode [6,

formed of a thin plate 'havingfperforations lxl,ra sectionron line .6--6.of..:]5igurea5. being. depicted in-Figure 6. It will be understood: thisFplate cathode v l6 maybe of any"'suitable.materialias hereinfurther explained and can be bent'on itself to provide a large working surface, similarly to-the way the' cathode I5 is formed. As seen in Figures-4 and 6, thesilver'indicated as l8,will form around the wires or spacers and gradually buildup as a smooth surface; At all times, the

perforationsl l will -allow the bath or solution to "wash through and thus avoidhaving trapped pockets of'bath or solution.

Figure 7 is a modified form of cathode l9 consisting of an arrangement of parallel rods or wires 20. The silver as extracted from the bath orsolution will build upsrnoothly around these rods or wires 20 similarly to the manner in which it is collected on thecathodes l5 and IE5 It will be understood thecurrent flows from the anode to the cathode through the photographic bath to thereby cause posit on the cathode.

As will be notedfrom the shown a tank which may be of any usual shape, an anode suspendedat one side and a cathode at the oppositeside; -Diagrammatically I have represented an electrical: connection and. source of the silver'to de-.

favorable results are had with the cathode being made of Monel metal or of stainless steel, but it may be made of silver or copper-nickel alloy or other metals and metal alloys or any materials which will conduct an electric current and which are not subject to attack by the photographic solution or bath. The most important feature of my cathode is that it shall be formed with or provided with perforations throughout, as for example, it may be in the form of a wire screen mesh. I have used such mesh having 256 openings per square inch of area with great success.

The cathode does not however necessarilyvhave to be a screen mesh type. It may be a solid plate having numerous closely spaced openings.

It is essential to have as great a surface area slower its fixing action. Consequently, it is desirable to remove the silver continuously as by as possible in the cathode for the collection thereon and deposit therein of the silver, but the overall dimensions of the cathode must be kept within such limits as will not interfer with the ordinary work going on in the use of thebath. By using the wire screen mesh ora thin perforated plate type of cathode, I can foldthe cathode so as to have a number of layersof surface-"area giving a large'working. surface to the cathode within actually a small space...

A cathode formed of what may for convenience be described as a bundle of parallel rods or wires can also be used and thereby obtain large working surface'area. 7

In whatever form the cathode may be' made, the essentialfeature is to provide the large surface area for'the' metal to deposit thereon and to "have'spa-ces forthe solution to migrate or circulate through freely, and to obtain a dense, adherent plating of silver, free from sponginess. Such a requirement is ideally met by thetthin perforatedplate or wire mesh screen hereinidescribed; I find that in the case of the wire mesh or bundles ofrods or wires, the silver' "builds up around thesurface of each wire until "the adjacent wires are so coated that the openings of the mesh are nearly closed. .The brideing pieces between the'holes, in the case of the perforated sheet build up with silver coating until the sanie result i'siobtained. The essential 'pointzis to permit free migration or circulation of the-solution through the cathode to avoid entrapment. The folded screen type of cathode possibly causes a' relatively rapid movement of silver ions within, the open mesh structure of the. screen in the same manner that a grid in an electronic tube accelerates the flow of electrons. within such tube. "The result of such increased ionic mobility is ultimately to produce the same effect as if the solution itself were agitated, as proposed by others, namely to secure a smooth adherent deposit of silver. Myscreen cathode accomplishes this without the use of any additional mechanisms. It may "also be that the cathode arrangement, in, which a multitude of discrete portions of the solution are substantially but not completely encompassedby electrode surfaces-causes silver ions in saidportions to move rapidlyto the nearest "cathode surface and thusrapidly change the specific gravity of those portions and induce circulation or migration .of solution through the cathode. In any event,'as previously pointed 0111;,1'110 positive agitation of the solution is necessary, the electrolysis being carried cum a normally substantially quiescent solution; While the, ordinaryv work going'lon in the use of the bath will at times include processing of filmand consequently some agitation of the .bath, such agitation is not nee my process which is a continuous one and thus keep the solution in a most favorable condition at all times.

It is essential to have an electric current density of such low value that it does not cause the tographic fixing solution, comprising the steps solution to disintegrate and break up into its chemical components.

In order to obtain sufficient current throughput for effective desilverization of heavy duty fixing baths without damage to the bath, others have proposed completely filling a separate tank with a plurality of'stainless steel cathodes, and circulating; the hypo through this extra tank. This is usually undesirable and is obviated in my apparatus since I have the necessary surface in a compact form by the use of the aforementionecl mesh; v My system will'operate without any particular agitation of the solution as proposed byothers and requires no special attention after the current has been adjusted to the proper value, an extremely desirable feature. v

A difficulty with using plain flatsheets, not

perforated, as cathodes is that they collect the silver on their surfaces, in the form of small lumps or granular nodules and these atthe slightest-disturbance of the sheet will fall off into the tank.

v I have found that with the use of types of cathodes and with one volt to one and two-tenths volts direct current at one-fourth ampere to-one .1. The method of. removing silver froma phooi}: establishing a substantially quiescent body o'fsaid fixing solution,1.establishing an anode zone'within but adjacent one side of said, body and a cathode zone consisting essentially of an electrodepositing zone-of substantial depth within but adjacent the opposite side of .saidbod'y, passing an electric current from said anode zone to said cathode zone, causingsaid current/to leave said solution in said cathode zone through a multitude of'ion collecting cathode surfaces therein facing in all directions, said surfaces being so; arranged as to substantially but not toward one of saidsurfaces and thus effect rapid removal ofsaid silver ions from said portions. 1

2. The method; of claiml wherein said electric current is passed from said anode zone to said cathode zone at a voltage of from one to one and two tenths volts.

3. The method of claim 1 wherein said surfaces are defined by a plurality of closely adjacent layers of wire mesh.

HENRY O. WILLIER.

REFERENCES CITED The following references are of record in the file of this patent:

Number lNumber Australia Mar. 26, 1941 

1. THE METHOD OF REMOVING SILVER FROM A PHOTOGRAPHIC FIXING SOLUTION, COMPRISING THE STEPS OF: ESTABLISHING A SUBSTANTIALLY QUIESCENT BODY OF SAID FIXING SOLUTION, ESTABLISHING AN ANODE ZONE WITHIN BUT ADJACENT ONE SIDE OF SAID BODY AND A CATHODE ZONE CONSISTING ESSENTIALLY OF AN ELECTRODEPOSITING ZONE OF SUBSTANTIAL DEPTH WITHIN BUT ADJACENT THE OPPOSITE SIDE OF SAID BODY, PASSING AN ELECTRIC CURRENT FROM SAID ANODE ZONE TO SAID CATHODE ZONE, CAUSING SAID CURRENT TO LEAVE SAID SOLUTION IN SAID CATHODE ZONE THROUGH 